This invention relates to a hydraulic piston cylinder device comprising a cylinder, a piston dividing the cylinder into two cylinder spaces, and valve elements between the cylinder spaces and the respective pipes for actuation by a pressure medium.
When this kind of piston cylinder device is being used and the external actuating pressure stops, both valve elements are closed by springs whereby the piston cannot be moved by any external force in either direction because liquid cannot escape from either of the cylinder spaces. It would be sometimes necessary, however, that the pressing or tractive force of the cylinder is maintained even when the external actuating pressure is no longer present. For this purpose it is known to use a pressure accumulator in the hydraulic system. In most cases a separate pressure accumulator connected with hoses is sufficient, but sometimes such accumulator does not provide the required reliability. For example breaking of the hose between the piston cylinder and the accumulator stops the function of the accumulator. There are some events where such uncertain factor cannot be allowed, e.g.:
A retaining centralizer of the drill rod whereby the pressure accumulator prevents the dropping of drill rods into the hole if the external working pressure stops for some reason,
Different kinds of support devices, for instance the top supports of wagon drills,
Some brake systems.